16 Biodiversity and the Balance of Nature S.L. Pimm 16.1 What Biodiversity Is Good for As we destroy biological diversity, what else are we doing to the environ­ ment, what is being changed, and how will those changes affect us? One part of the answers to these questions is provided by Lawton and Brown's consideration of redundancy (Chap. 12). Part of what the environment does for us involves "ecosystem services" - the movement of energy and nutrients through the air, water, and land, and through the food chains (Ehrlich, Foreword). Just how much biological diversity we need to keep the movement at approximately natural levels is a question of critical im­ portance. Nonetheless, it is not a question that is commonly asked. A major synthesis of theories on the dynamics of nutrient cycling (DeAngelis 1991) devotes little space to the consequences of changes in the numbers of species per trophic level: It is the number of trophic levels that receives the atten­ tion. One might well conclude that, over broad limits, ecosystem services will continue to be provided, so long as there are some plants, some animals, some decomposers, and so on. Lawton and Brown conclude that numerous species are redundant. One cannot be confident about this conclusion, for the evidence one way or another is still very limited. Even if it were correct, it would not provide a complete answer to the questions of the value of biological diversity. The conclusion only deals with the average movement of material; ecologists have long recognised that diversity may be important in determining the variability and persistence of ecological processes and their resistance to external changes. Following the loss of diversity, one often hears ideas such as: the loss threatens ecological integrity; the environment becomes more fragile as diversity decreases; ecosystems have some capacity to resist change or to recover from it, that is, there is some balance of nature or stability, and this balance is affected by the loss of diversity. So what do all these terms mean and what scientific principles do they encompass? One illustration of these concerns involves the fisheries in the Great Lakes of North America. For over a century and a half, these lakes have been subjected to pollution, intense fishing, the loss of several native species, E.-D. Schulze et al. (eds.), Biodiversity and Ecosystem Function © Springer-Verlag Berlin Heidelberg 1994 348 S.L. Pimm and the deliberate and accidental introduction of other fish species. For example, the Lake Erie fishery between 1920 and 1950 was dominated by blue pike (33% of the yield by weight), lake herring (14%), and yellow perch (15%); by 1979-1983 the first two species had been lost, and rainbow smelt accounted for 56% of the yield (Loftus et al. 1987). In contrast, the average total landings increased slightly (about 30%). Similar figures are noted for the other lakes. Simply, the secondary productivity of these ecosystems has changed remarkable little during a period when the com­ position changed greatly. Considerable experience with the lakes and their fisheries (see the papers in Christie and Spangler 1987) reveals that there is no comfort in the similarity of the total yields. In part, this is because the blue pike are worth much more per kilogram than rainbow smelt. (In reality, the species do matter; we cannot reduce all ecology to flows of nutrients.) More important is the sense that the ecosystems in the lakes no longer function in the same way as they did in the past. It is not always clear what these changes in function are, but they are of considerable concern. They involve the internal dynamics of the lakes and they are independent of such summary statistics as overall yield. So just what is diversity good for? It seems to have something to do with what we call stability. Ecological stability has a history, and I want to review it briefly to set the context of what follows. 16.2 A History of Ecological Stability In his 1958 book, The Ecology of Invasions by Animals and Plants, Elton wrote: I will now try and set out some of the evidence that the balance of relatively simple communities of plants and animals is more easily upset than that of richer ones; that [the balance] is more subject to destructive oscillations in populations, especially of animals, and more vulnerable to invasions.... For if this can be shown to be anywhere near the truth, it will have to be admitted that there is something very dangerous about handling cultivated land as we handle it now, and [it will be] even more dangerous if we continue down the present road of 'simplification for efficiency'. Elton then discuses the evidence for his assertion that simple communities are less stable than complex ones. His first evidence comes from "mathe­ matical speculation about population dynamics." Simple models of a pred­ ator and its prey or a single host and its parasite have population densities that "fluctuate in numbers considerably." Such fluctuations would mean that in simple communities, populations would "never have constant population levels, but would be subject to periodic 'outbreaks' of each species". Elton's second line of evidence was from laboratory experiments that made the Biodiversity and the Balance of Nature 349 same point. The third piece of evidence is that the natural habitats on small islands seem to be much more vulnerable to invading species than those of continents. His fourth argument was that "invasions and pest oubreaks most often occur on cultivated or planted land - that is, habitats and communities very much simplified by man". His fifth argument was similar. Insect out­ breaks, Elton claimed, were a feature of simple temperate forests but not of complex tropical forests. His sixth argument seems to recapitulate the earlier ones: insect pests are more likely to invade and become a nuisance in orchards, especially following spraying designed to eliminate the pests. Elton's arguments have received extensive discussion. The models and the laboratory experiments available to Elton looked only at simple systems: Models of, or experiments with, complex systems were not available as a control. The data on invasions were compelling, but comparative studies of population fluctuations were nonexistent. Moreover, Elton discussed a range of very different phenomena: population variability, the potential abundance of a species, the rate at which a pest species might increase in numbers after the removal of its predators, how readily communities may be invaded, and the extent of damage the invading species cause. Yet this range is perhaps the most remarkable part of Elton's argument. Ecologists should be thinking about different kinds of stability: population variability, population recovery, ease of invasion, and consequences of invasion. All are ways of characterizing population and community changes to facilitate comparisons. Moreover, Elton considers a factor called "complexity" that should determine "stability" and the importance of their interdependence. Notice something else: the effect of humans is to destroy stability, and with the loss of stability comes the loss of species, and so less complexity. Systems that are less complex are less stable, more likely to lose species, and so on. 16.2.1 Controversy The relationships between stability and complexity have generated a large literature (Pimm 1982, 1984). Controversy arose because of the many meanings of "stability" and "complexity" (Pimm 1984). Elton and more recent theoretical and empirical ecologists have used the word "stability" to mean at least five different things (Pimm 1984, 1991b). Theoreticians have considered stability in its strict mathematical sense, where systems are either stable or not. So given May's (1974) argument that what we observe most often in nature will be stable systems, the theoretical results make predictions about the kind of communities we observe and those that we do not. The other four definitions of stability do permit comparisons of dif­ ferent existing systems, however, and these were the concerns of empirical ecologists. 350 S.L. Pimm 1.2 1.0 E .;:::J 0.8 .c 0.6 ::J 0'" 0.4 Q) E 0.2 o... -c: 0.0 f----;:=---=---=:a==.---.Ik-~tr_.....,.r___...-...._- 0 :;:; -0.2 III .~ -0.4 0 -0.6 0 2 4 6 8 10 Time (t) 140 40000 .c l/I 120 ::l 30000 III .c ::l t- iii Cl Q. C 100 ::l 0 al C/) 20000 "0 "0 80 ?: ?: 0; ·iii c c .. .. 10000 Q Q 60 40 0 10 20 Year (starting 1960 for Song Thrush, 1890 for Bupalus) Fig. 16.1. Above, Two model populations returning to equilibrium density (set at zero) from the same initial displacement at +0.5. One population (open triangles) is much less resilient than the other (closed triangles) and returns more slowly to equilibrium. Below, Extent of year-to-year variabilities for different species: song thrush (open squares) in English farmlands (units of density relative to 100 in 1966) and the moth Bupalus (solid circles) in German forests (numbers per hectare of forest floor) To complicate matters further, different studies looked at "stabilities" at different levels of ecological organization. Theoretical studies of whether interacting sets of species will be stable consider whether the densities of all those species return to equilibrium or not. To ask how long a community persists before it is invaded is to ask how long the community composition lasts. Such a discussion ignores fluctuations in the abundance of species and looks only at the species list itself. In contrast, McNaughton (1977; Chap. 17) placed his emphasis on how much the total biomass changed and not on which species were involved in the changes. Therefore, we can discuss the stability of populations, community composition, or community biomass.